A traditional rotary lawnmower cuts grass at a pre-set cutting level. The cut grass normally stays on the fresh-cut lawn and will then be visible in a distinct and often unpleasant manner. For this reason, the grass is often collected and carried away from the lawn. This collection can be performed, for instance, by the cut grass being directly brought into a collecting container or by the cut grass being raked together after the cutting of the lawn. This is, however, disadvantageous since nourishment is removed from the lawn.
To solve this problem systems for disintegrating the grass have been developed. Hence the grass can be restored to the lawn in a way that is appealing to the eye since the disintegrated grass can penetrate into the lawn and thus become less noticeable. In addition, the cut grass degrades faster when it is cut into pieces.
One type of such rotary lawnmowers maintains the cut grass in the cover of the rotor and causes chopping knives to chop it, after which the small pieces fall down on the lawn. According to a second type of rotary lawnmower, the cut grass is put on the lawn, after which the knives of the rotary lawnmower engage the cut grass once again and chop it into small pieces. The problem of these types is that the cutting quality depends on the cutting conditions. In many cases, the grass which is cut from the lawn is only beaten around by the chopping knives without being hit and thus chopped, and if the grass should happen to be wet, the cut grass tends to be rubbed rather than chopped by the rotor knives.
A third type of rotary lawnmower, which solves the above problems, comprises a rotor which theoretically operates at a plurality of cutting levels. In this case, the rotor is arranged so as to first operate at the highest cutting level, and then successively down to the lowest. The grass is thus cut by steps, only a small piece of each blade of grass being cut in each step. Examples of such rotary lawnmowers are disclosed in FR 2,468,289, U.S. Pat. No. 4,351,144 and WO 96/29853. Since the grass is cut successively down to the desired height, so-called topping, the above-mentioned "beating" problem is eliminated. There is, however, a disadvantage of this type of lawnmower as well. It has appeared in practical application that the rotor does not operate in the intended manner. If the grass is high, the parts of the rotor which operate at the higher cutting levels will not be able to top the grass because when the lawnmower is travelling, the grass is first applied against the cover of the rotor, whereby the grass is bent forwards. Subsequently, when the grass thus bent forwards enters the rotor, this bending forwards of the grass is maintained and enhanced by the flow of air caused by the rotor. Consequently, at these higher cutting levels the grass escapes being hit and thus being cut by the rotor. Hence, it is only the intermediate and low parts of the rotor that cut the grass in a fairly efficient manner, which implies that a large piece of each blade of grass will be cut off. This results in the inconvenience of having an unpleasant accumulation of cut grass on the fresh-cut lawn.
The object of the present invention is to provide a rotor assembly and a rotary lawnmower of the above-mentioned second type, which secure a satisfactory disintegration of the grass, even if the grass is high.